Plunger type master cylinder

ABSTRACT

To provide a plunger type master cylinder capable of reducing invalid stroke and preventing residual pressure without impairing durability of a seal ling ( 70 ). A piston ( 91 ) of the master cylinder has a recess ( 93 ) at its outer peripheral surface and a relief port ( 900 ) in its recess ( 93 ). The seal ring ( 70 ) has a seal coupling part ( 70   s ) on the inner periphery of the distal end part of an inner peripheral lip ( 701 ). The seal coupling part ( 70   s ) is located at the opening of the relief portion ( 900 ) on the piston ( 91 ) side when the brake is in a non-operating position. The distance between said seal coupling part ( 70   s ) and said relief port ( 900 ) is short. The seal ring ( 70 ) additionally has first and second projection parts ( 7006, 7002 ) on the inner peripheral surface of the inner peripheral lip ( 701 ). By this, a flow passage for working fluid can be obtained on the inner periphery side of the inner peripheral lip ( 701 ).

This application is a 371 of PCT/JP2004/005725 filed on Apr. 21, 2004,published on Nov. 4, 2004 under publication number WO 2004/094208 A1which claims priority benefits from Japanese Patent Application Number2003-118943 filed Apr. 23, 2003 and Japanese Patent Application Number2003-118944 filed Apr. 23, 2003 and Japanese Patent Application Number2003-118945 filed Apr. 23, 2003.

TECHNICAL FIELD

This invention relates to a plunger type master cylinder used in ahydraulic brake system for vehicles, and the like, and more particularlyto the technique of a master cylinder suited for brake control (forexample, automatic brake control for controlling the attitude of avehicle, or traction control for controlling the braking force of adrive wheel at the time of acceleration of a vehicle, and the like)which is performed separately from the manual brake control made by adriver.

BACKGROUND ART

A plunger type master cylinder used for the purposes as mentioned abovehas a relief port formed in a piston capable of moving in an axialdirection within a cylinder bore of a cylinder housing. It is said thata seal ring for opening and closing the relief port is high indurability.

As technical problems to be solved for such a plunger type mastercylinder, firstly, there is such a problem as how to reduce the invalidstroke caused by braking operation and secondly, there is such a problemas how to prevent the residue of a brake fluid pressure (so-calledresidual pressure) in a fluid pressure chamber during non-operation ofthe brake. As a related art, there can be listed, for example, theOfficial Gazette of DE 19,536,325A1 (see especially its FIG. 1). In thisrelated art, a technique for reducing the invalid stroke which belongsto the first-mentioned problem to be solved is disclosed, in which apiston (13) including a relief port (20) is provided at its tip partwith a reduced-diameter part (40), and a seal ring (11) is arranged atthe reduced-diameter part (40). As another related art, there can alsobe listed the Official Gazette of Japanese Patent Application Laid-OpenNo. 2000-108878 (see especially its FIG. 2). This related art alsodiscloses a technique for reducing the first-mentioned invalid stroke,in which a piston (13) is provided with a control tapered-surface (86)which is formed on the outer peripheral surface of a piston (13) at therear side of an opening part of a relief port (56). The inner peripheryof a cup seal (46) can be sealed by utilizing this controltapered-surface (86). Moreover, Official Gazette of Japanese PatentApplication Laid-Open No. 2001-146157 (see especially its FIG. 2)discloses a technique for preventing the second-mentioned residualpressure, in which a relief port (4 c) is formed at a piston (4) and asafety hole (4 e) is disposed at a rear side of the relief port, so thatpressure in a fluid pressure chamber can positively be relieved throughthe safety hole.

PROBLEM TO BE SOLVED BY THE INVENTION

In the stage for developing the technique for solving theabove-mentioned two problems, i.e., reduction of invalid stroke andprevention of residual pressure, the inventors have noticed that theusual plunger type master cylinders including the above-mentionedrelated arts have a certain common point. What is noticed by theinventors is a common point in positional relation between a piston anda seal ring for sealing its outer periphery. That is, when the mastercylinder is in a non-operating position, at least the tip part of aninner peripheral lip of the seal ring is located at the front side ofthe relief port as viewed in the axial direction. In contrast, when themaster cylinder is in an operating position, the inner peripheral lip ofthe seal ring is located at the rear side of the relief port. Therefore,when the master cylinder repeats the shifting motion between thenon-operating position and the operating position, the inner peripherallip of the seal ring, at least the tip part of the inner peripheral liptraverses both the second edge at the front side of the opening of therelief port and the first edge at the rear side of the opening of therelief port. The “front side” of the relief port refers, as viewed inthe axial direction, to the side which is nearer to the fluid pressurechamber, and the rear side, to the side which is nearer to the openingof the cylinder bore, respectively.

The relief port which is formed at the piston, is a hole having adiameter, for example, of about 2 mm. The edge of the opening part ofthe relief port is somewhat square due to hole machining. Therefore, therepeated traversing motion across such an edge as just mentioned wouldresult in deterioration of the lip of the seal ring, which is composedof rubber material, due to aging fatigue. This is liable to impairdurability of the seal ring. Although it is a good idea to prevent theresidual pressure by forming a safety hole at the piston, the holemachining applied to the piston is preferably reduced as much aspossible.

It is, therefore, an object of the present invention to provide aplunger type master cylinder which is capable of solving theabove-mentioned two problems, i.e., reduction of invalid stroke andprevention of residual pressure, without impairing durability of theseal ring.

It is another object of the present invention to provide a techniquecapable of effectively conducting the prevention of residual pressure,while reducing, as much as possible, hole machining applied to thepiston.

Other objects of the present invention will become manifest from thefollowing description.

DISCLOSURE OF THE INVENTION

The present invention adopts such an idea that the various problems ofenhancement of durability of the seal ring, reduction of invalid strokeand prevention of residual pressure are effectively solved by arrangingthe inner peripheral lip of the seal ring and the relief port on thepiston side in a predetermined positional relation. The fundamentalfeatures of this idea reside in the following features 1, 2 and 3, andalso reside, as additional features, in the features 4 and 5 as laterdescribed.

-   feature 1. A recess is formed on an outer peripheral surface of the    piston ranging, when axially viewed, at least from a second edge    (edge on the side near the fluid pressure chamber side) of the    opening of the relief port to a first edge (edge on the side remote    from the fluid pressure chamber side), further ranging to an area    away by a predetermined distance from the first edge of the opening    of the relief port toward the first end part side, and further    ranging over the entire length in a peripheral direction of the    piston.-   feature 2. The inner lip of the seal ring is smaller in diameter at    a distal end part as a free end thereof than at a root part on the    base part side, thereby the seal ring has a seal coupling part    situated at an inner periphery of the distal end part of the inner    lip which is small in diameter which is seal-coupled to an outer    peripheral wall surface of the piston and adapted to seal-coupled to    an outer peripheral wall surface of the piston, and moreover, a void    for communicating the relief port with the communication passage    together with the recess is formed between an inner periphery of the    inner lip ranging from the seal coupling part to the root part.-   feature 3. The seal coupling part of the distal end part of the    inner lip of the seal ring is located at the opening part of the    relief port on the side of the piston when the master cylinder is in    a non-operating position.

Owing to the recess formed in the outer periphery of the piston underfeature 1, and the seal coupling part of the seal ring and the voidbetween the inner periphery of the inner lip and the outer peripheralwall surface of the piston under feature 2, the fluid pressure chambercan be communicated with the reservoir side when the master cylinder isin a non-operating position. Owing to the unique and characteristicarrangement of the seal coupling part (or the distal end part of theinner lip of the seal ring) when the master cylinder is in anon-operating position under feature 3, durability of the seal ring canbe enhanced and invalid stroke can be reduced. Particularly, thosefeatures 1 through 3 are of great help to solve three problems withoutimpairing the features which others have and by mutually cooperating.

Particularly, the distal end part of the inner lip of the seal ring islocated in the opening of the relief port when the master cylinder is ina non-operating position. This means that the seal coupling part of theseal ring traverses only the edge on the side of the first edge of allthe entire edge of the opening of the relief port in accordance withnon-operation and operation of the master cylinder. Therefore, therepeat of non-operation and operation reduces the number of times forthe seal coupling part to traverse the edge into a half. It can beexpected that durability of the seal ring is enhanced to that extent.

The bore of the relief port (axial diameter of the opening) is, forexample, about 2 mm. In contrast, the relative movement (axial distance)between the seal ring and the piston caused by operation of the mastercylinder varies depending on how brake is actuated. However, thisvariation in relative movement is in the range from, for example, a fewmillimeters to 10 mm in quantity of axial stroke. Therefore, the sealcoupling part of the seal ring is located at the outer periphery of thepiston passing over the recess when the master cylinder is in acondition of normal operation, but the seal coupling part is sometimeslocated in the recess when the brake is actuated lightly.

The recess formed at the outer periphery of the piston ranges over theentire length in the peripheral direction of the piston but it ranges ina limited region in the vicinity of the relief port in the axialdirection. Thus, the piston has enlarged-diameter parts which are eachlocated in a front and a rear position of the recess. In a preferredembodiment, the both front and rear parts of the recess of the pistonare guided by a piston guide part on the cylinder housing side. In casethe piston guide part is arranged at the front side of the recess, thesealing force in the peripheral direction of the seal ring caneffectively be uniformed because the piston guide part is in a positionnear the seal coupling part of the seal ring which is brought into theopening of the seal ring.

Since a void is formed between the inner periphery of the inner lip ofthe seal ring and the outer peripheral wall surface of the piston, theresidual pressure can be released to the reservoir side through the voidand the communication passage when the master cylinder is in anon-operating position. In a normal case, residual pressure can beprevented by this. However, it can be imagined that when the workingfluid on the reservoir side is supplied to the wheel cylinder side ofthe brake device through the fluid pressure chamber under pump actionwhen the automatic brake is in an operating position, the inner lip ofthe seal ring is oscillatingly deformed due to fluctuation of fluidpressure of the working fluid, etc. and the void which is turned out tobe a passage, is reduced, thereby disabling to supply the fluidsufficiently. In order to effectively obtain such fluid supply property,in a preferred embodiment of the present invention, the seal ringfurther has the following feature(s) 4 and/or 5.

-   feature 4. The seal ring comprises a plurality of first projection    parts formed on an inner peripheral surface of the inner lip between    the root part and the seal coupling part and bulged from the inner    peripheral surface of the inner lip toward the outer peripheral    surface of the piston, and the first projection parts are spacedly    arranged in the peripheral direction, thereby forming an axial flow    passage between the first projection parts which are adjacent to    each other in the peripheral direction.-   feature 5. The seal ring is deformable such that the inner lip is    oscillated radially inward and outward about the root part, the base    part, the base part is provided at an inside surface on the side    where the inner and outer lips extend with a swollen part for    enlarging the axial thickness of the base part on the inner lip side    compared with the outer lip side, and the center of oscillation of    the inner lip is located in a position nearer to the second end part    side in the axial direction than in a case where no swollen part is    provided.

As a modified embodiment having the feature 4, the second projectionparts are provided parallel to the first projection parts (projectionparts are provided in two row between the root part and the sealcoupling part). Owing to this arrangement, the void on the innerperiphery side of the inner lip can more surely be obtained. Althoughthe features 4 and 5 can individually effectively act in obtaining fluidsupply property, an embodiment having both the features 4 and 5 is morepreferable.

In the master cylinder of this invention, it is accepted that in orderto attach the seal ring to the cylinder housing side, a slide ring isinserted in the inner periphery of the cylinder bore and one side of theseal ring is supported by one end of the slide ring. It is morepreferable that an attachment groove for attaching the seal ring isprovided directly to the inner peripheral wall surface of the cylinderbore of the cylinder housing. Owing to this arrangement, the number ofcomponent parts can be reduced, and the positional relation between therelief port on the piston side and the seal ring on the cylinder housingside can established with higher precision. The seal ring is disposed inthe attachment groove with its base part sunk in the attachment grooveand with the seal coupling part on the inner periphery of the distal endpart of the inner peripheral lip allowed to project outward from theattachment groove.

The present invention can be applied not only to the tandem type butalso to the single type. In case the present invention is applied to thetandem type, it can be applied to both of the primary side and thesecondary side (i.e., to both side in the plunger type) or only to oneof the primary side and the secondary side (i.e., to one side in theplunger type).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view taken on the axis of a tandem plunger typemaster cylinder according to one embodiment of the present invention.

FIG. 2 is an enlarged sectional view showing the area in the vicinity ofa relief port of the master cylinder of FIG. 1, in which the brake is ina non-operating position.

FIG. 3 is, likewise, an enlarged sectional view of the area in thevicinity of a relief port of the master cylinder of FIG. 1, showing astate in the midst of stoke in accordance with braking operation.

FIG. 4 is, likewise, an enlarged sectional view of the area in thevicinity of a relief port of the master cylinder of FIG. 1, showing astate of terminal end of stroke in accordance with braking operation.

FIG. 5A is a sectional view showing a first embodiment of a seal ringemployed in the present invention.

FIG. 5B is a view, as viewed in the axial direction, of the seal ring ofFIG. 5A.

FIG. 6A is a sectional view showing a second embodiment of a seal ringemployed in the present invention.

FIG. 6B is a view, as viewed in the axial direction, of the seal ring ofFIG. 6A.

FIG. 7 is an enlarged sectional view showing a third embodiment of aseal ring employed in the present invention.

FIG. 8 is an enlarged sectional view showing a fourth embodiment of aseal ring employed in the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is one embodiment of a tandem plunger type master cylinder towhich the present invention is applied. The tandem master cylinder 10comprises a primary part 101 and a secondary part 102 which are mutuallyindependent. In the illustrated embodiment, the way of thinkingaccording to the present invention is applied to those two parts 101,102.

First, referring to FIG. 1, an overall construction of the tandem mastercylinder 10 is made manifest. An outer shell of the master cylinder 10is a cylinder housing 20 made of an aluminum alloy. The cylinder housing20 is provided at an upper part thereof with a boss part 201 forsupporting a reservoir 30 and at its inside with a cylinder bore 22axially extending from a first end part 20 h which is open, to a secondend part 20 b which is closed. The boss part 201 is a part forsupporting the reservoir 30 which reserves therein working fluid.Nipples 301, 302 of the reservoir 30 are fitted for connection to theinner side of the boss part 201. Flow passages communicated with thecorresponding nipples 301, 302 are for the primary side and for thesecondary side, respectively, and they are mutually independent.

When the cylinder bore 22 is axially viewed in a direction from thefirst end part 20 h which is open, to the second end part 20 b which isclosed, there are sequentially provided, in order, a firstreduced-diameter bore 221 s which is located at an area near the firstend part 20 h, a first enlarged-diameter bore 221 b which is located ata deep side of the first reduced-diameter bore 221 s and which has alarger diameter than the first reduced-diameter bore 221 s, a secondreduced-diameter bore 222 s which is located at even a deeper side ofthe first enlarged-diameter bore 221 b and at an inner side of a bosspart 201 and which has the same diameter as the first reduced-diameterbore 221 s, and a second enlarged-diameter bore 222 b which is locatedat the deepest side of the cylinder bore 22 and which has the samediameter as the first enlarged-diameter bore 221 b. The inner parts ofthe first and second enlarged-diameter bores 221 b, 222 b each having alarge diameter constitute a fluid pressure chamber. Since the componentbores 221 b, 222 b are large in diameter, a fluid pressure chamberhaving a comparatively large volume can be defined while miniaturizingthe cylinder housing 20. On the other hand, those parts of the first andsecond reduced-diameter bores 221 s, 222 s each having a small diameterare areas where an attachment groove for attaching a seal ring is formedand which also constitute a piston guide part for guiding the movementof a piston. Since they are located in the innermost periphery, thoseparts of the first and second reduced-diameter bores 221 s, 222 s caneasily be machined and surface treated in inner peripheral wall surface,thereby ensuring them to have a mode suited for guide function. Ofcourse, in those parts constituting the cylinder bore 22, the mutuallyadjacent parts are connected to each other with a taper having acomparatively gentle angle of inclination.

Should attention be paid to those parts of the first and secondreduced-diameter bores 221 s, 222 s, those parts are provided with twoseal ring attachment grooves 40, 42 which are axially arranged with adistance. Between the attachment grooves 40, 42, there is acommunication passage (or passage constituting a part of it) which is incommunication with the reservoir 30. A communication passage 52 of thesecondary part 102 includes a ring groove 522 boring the inner peripheryof the cylinder bore 22 over one round, and a through passage 524penetrating through the wall of the cylinder housing 20 andintercommunicating the ring groove 522 and the inner side of the bosspart 201. The communication passage 50 of the primary part 101 includes,in addition to the ring groove 502 and the through passage 504 as in thesecondary part 102, a reservoir communication bore 506 extendingparallel to the cylinder bore 22 and adapted to intercommunicate thering groove 502 and the inner side of the boss part 201. It is a blindplug 508 that chokes the opening part on the first end part 20 h side ofthe reservoir communication bore 506. The cylinder housing 20surrounding the reservoir communication bore 506 is provided at an outerperiphery thereof with an attachment flange 203 for attaching the mastercylinder 10 to a booster (not shown). A top-like valve element 60 on theinner side of the boss part 201 is made from a resin material floatingon working fluid, and it includes a throttle passage 602 on its uppersurface. The valve element 60 constitutes a valve device together with asleeve-like valve seat component element 66 which is disposed at theinner side of the boss part 201. This valve device is located at amid-way of a passage intercommunicating a fluid pressure chamber of themaster cylinder 10 and the reservoir 30 and has such a function as tocommunicate the passage in a cut-off state or throttling state.

The seal ring attachment grooves 40, 42 supports seal rings 70, 72 whichare each composed of a cup-type seal and which are attached thereto,respectively. A secondary return spring 82, a secondary piston 92, aprimary return spring 81 and a primary piston 91 are sequentiallyinserted in the cylinder bore 22 in order from the deep side toward theopening. By this, the secondary piston 92 defines a second fluidpressure chamber 1020 at the deep side of the cylinder bore 22 togetherwith the seal ring 72 at the part of the second reduced-diameter bore222 s, and the primary piston 91 defines a first fluid pressure chamber1010 between the primary piston 91 and the secondary piston 92 togetherwith the seal ring 72 at the part of the first reduced-diameter bore 221s. The primary and secondary return springs 81, 82 are caged by a pairof spring retainers 83, 84 and a rod 85, respectively, thereby thepistons 91, 92 are prevented from jumping out of the cylinder bore 22.Although not shown, those parts of the first and second reduced-diameterbores 221 s, 222 s facing the fluid pressure chambers 1010, 1020,respectively, are provided with an output port for outputting a fluidpressure to a brake circuit. In order to smoothly intercommunicate theoutput port and the fluid pressure chambers even in the case the mastercylinder 10 is in a non-operating position, a groove extending in theaxial direction is preferably formed (see U.S. Pat. No. 4,524,585 for anidea for forming a groove) in an inner wall part of the cylinder bore 22where the first and second reduced-diameter bores 221 s, 222 s areformed. As a groove extending in the axial direction, a helical grooveis preferred.

The secondary piston 92 and the primary piston 91 include circularcylindrical parts 920, 910, respectively, each having an internal space.The circular cylindrical parts 920, 910 are each provided with aplurality of relief ports 900 penetrating through the circularcylindrical walls thereof. Each relief port 900 has a circular shape insection and is, for example, 2 mm in diameter. The numbers of the reliefports 900 are, normally, multiples of 4 (for example, 4, 8, 16 or 32),and 4 for each piston 92 (or 91) in this embodiment. Those four reliefports 900 are mutually equidistantly arranged along a circumferencewhich is orthogonal to the axis of each piston 92 (or 91).

The master cylinder 10 has features in construction of the area in thevicinity of the relief port 900. FIGS. 2 through 4 are enlarged viewsshowing the area in the vicinity of the relief port 900. FIG. 2 is aview showing a state in which the brake (i.e., brake actuated by adriver) is in a non-operating position, FIG. 3 is a view showing a statein the midst of stroke in accordance with braking operation, and FIG. 4is a view showing a state of terminal end of stroke in accordance withbraking operation. The features of the master cylinder 10 will now bedescribed with reference to FIG. 2 through 4, as well as FIG. 1.

Each piston 91 (or 92) has a ring-like recess 93 formed, over one round,in its outer peripheral surface where the relief port 900 is open. Therecess 93 is, for example, about 0.2 to 0.3 mm in depth and for example,about 4 to 5 mm in axial width. The axial opposite end parts of therecess 93 are provided with tapered surfaces 951, 952, respectively,each having an angle of about 30 degrees. Through those tapered surfaces951, 952, the bottom of the recess 93 having a small diameter and theouter peripheral parts 911, 912; 921, 922 of the pistons 91, 91 eachhaving a large diameter are made continuous with each othercomparatively smoothly. One end (i.e., the tapered surface 952 nearer tothe second end part 20 b side) of the recess 93 is nearer to the secondend part 20 b from the second edge 902 of the opening of the relief port900. On the other hand, the other end (i.e., the tapered surface 951nearer to the first end part 20 h side) of the recess 93 is displaced bya predetermined distance to the first end part 20 h from the first edge901 of the opening of the relief port 900. The above-mentionedpredetermined distance is a distance capable of intercommunicating thecommunication passages 50, 52 and the relief port 900 without allowingthe recess 93 to increase the flow resistance. Although the recess 93has a certain length in the axial direction, this does not mean that thediameters are reduced to the distal end parts on the second end partside 20 b of the respective pistons 91, 92. The pistons 91, 92 aresmoothly guided in axial movement at least at two places, namely, thedistal end parts and a separate place axially away from the distal endparts passing over the recess 93. This smooth guiding also assists theeffective seal coupling between the pistons 91, 92 and the seal ring 70.Since the recess 93 is disposed over the entire length in the peripheraldirection of each piston 91 (or 92), the seal surface pressure in theperipheral direction of the seal ring 70 can be uniformed.

The seal ring 70 related with the relief port 900 is provided at theinner periphery of the distal end part of the inner lip 701 with a sealcoupling part 70 a which is seal coupled to the outer peripheral wallsurface of the piston 91 (or 92). A cup-type seal ring 70 is entirelymade from a rubber material, and it comprises a base part 700 which islarge in thickness, and inner and outer lips 701, 702 respectivelyextending from the inner and outer peripheries of the base part 700toward the fluid pressure chamber 1010 (or 1020) side. The outer lip 702is provided at an outer periphery thereof with a tightening margin 702b, and its diameter is generally equal from the root part on the basepart 700 side to the distal end part as a free end. In contrast, theinner lip 701 is smaller in diameter at the distal end part side as afree end than at the root part on the base part 700 side. Owing to thisarrangement, the inner lip 701 of the seal ring 70 is inclined forwardand downward from the root part toward the distal end part as a freeend. Owing to this feature, the seal ring 70 defines a ring-like void 75between the inner periphery of the inner lip 701 ranging from the sealcoupling part 70 s on the distal end part to the root part and the outerperipheral wall surface of the piston 91 (or 92). This void 75 functionsas an intermediate communication passage for intercommunicating therelief port 900 and the communication passage 50 (or 52).

In the master cylinder 10, the seal coupling part 70 s of the seal ring70 is arranged at the opening part of the relief port 900 axially on thepiston 91 (or 92) side when the master cylinder 10 is in a non-operatingposition (position shown in FIGS. 1 and 2). This means that the sealcoupling part 70 s is located between the second edge 902 and the firstedge 901 of the relief port 900. In order to enlarge the void 75, theseal coupling part 70 s is preferably arranged nearer to the second edge902, and in order to minimize invalid stroke of the brake, the sealcoupling part 70 s is preferably arranged nearer to the first edge 901.The size of the void 75 does not have any direct effect given at thetime of braking operation made by a driver (i.e., manual brakingoperation). However, the size of the void 75 has a direct effect to easysupply of working fluid at the time the working fluid on the reservoir30 side is supplied to the brake circuit through the relief port 900 andthe fluid pressure chamber 1010 (or 1020) under the external pump actionof the master cylinder 10 when the automatic braking operation isundergoing. In order to perform the automatic braking operationsmoothly, the void 75 should be set to a predetermined size or larger sothat the flow resistance is reduced at the time of supplying the workingfluid. The recess 93 formed in the outer periphery of the piston 90 (or92) is very effective for effectively enlarging the void 75 and reducingthe flow resistance of the working fluid. The circuit itself of theautomatic brake is known as shown in FIG. 7 of Japanese PatentApplication Laid-Open No. 2002-154420, for example.

The void 75 is also effective for preventing the residual pressure whenthe brake is in a non-operating position. It should be noted, however,that deformation, if any, of the inner lip 701 of the seal ring 70 mayinterfere the prevention of residual pressure. The reason is that thefluid pressure in the fluid pressure chamber 1010 (or 1020) acts on theback of the inner lip 701 facing the outer lip 702 to deform the innerlip 701 toward the first edge 901 side of the relief port 900, therebycreating a fear of reduction of the void 75.

Therefore, in the master cylinder 10, the form of the cylinder ring 70,particularly, the inner peripheral surface of the inner lip 701 facingthe outer peripheral surface of the piston 91 (or 92) is applied withsome elaboration. This elaboration includes an arrangement in whichbulged first projection parts are disposed at a plurality of placesmutually separated in the peripheral direction, thereby always obtainingan axial flow passage between every adjacent projection parts in theperipheral direction. Owing to a provision of such elaboration, therecan be obtained a flow passage of working fluid on the inner peripheryside of the inner lip 701 even if the inner lip 701 should be apt tostick to the outer peripheral surface of the piston 91 (or 92) when theautomatic brake is actuated (that is, when the automatic brake isactuated with the seal coupling part 70 s at the distal end part of theinner lip 701 located at the opening part of the relief port 900).

FIGS. 5A and 5B show an example in which a partly projecting chevronprojection 7006 is used as the first projection part. Six, for example,of such chevron projections 7006 may be equidistantly arranged in theperipheral direction. Those projections 7006 is located, when viewed inthe axial direction which is also the extending direction of the lip,between the root part on the base part 700 side and the seal couplingpart 70 s at the distal end. The root part or inner periphery on thebase part 700 side is provided with second protrusion parts 7002 whichare located at places, for example, six places as in the case with theprojections 7006, on the corner part facing the void 75 or piston 91 (or92) side which places are equidistantly separated in the peripheraldirection, and an axial groove (flow passage) 7003 is formed betweenevery adjacent second projection parts. In other words, the groove (flowpassage) 7003 is formed by notching the corner part of the base part700. Those grooves (flow passages) 7003 realize a flow of working fluidflowing from the relief port 900 side toward the communication passage50 (or 52) side, together with the flow passage formed between everyadjacent chevron projections 7006. Accordingly, residual pressure can beprevented from occurring. In order to surely intercommunicate theaxially adjacent grooves (flow passages) 7003 and the flow passageformed between every adjacent chevron projections 7006, the secondprojection part 7002 on the corner part and the chevron projection 7006disposed at its inner side may preferably displaced in the peripheraldirection.

FIGS. 6A and 6B show another example of the first projection part, inwhich record groove-like regions 7008 having grooves extending in theperipheral direction and banks arranged in parallel relation with thegrooves, are each used as the first projection part. Each recordgroove-like region 7008 is reduced in height compared with the chevronprojection 7006 and its area is enlarged. In the record groove-likeregion 7008, it is also possible to form the axial passage as in thechevron projection 7006, so that residual pressure can effectively beprevented from occurring.

When the brake is in a non-operating position as shown in FIG. 2, theseal coupling part 70 s of the seal ring 70 is located at the openingpart of the relief port 900 on the piston 91 (or 92) side. When thepiston 91 (or 92) is moved toward the second end part 20 b side inaccordance with the braking operation performed by a driver, the fluidpressure in the fluid pressure chamber 1010 (or 1020) starts to rise atthe time the seal coupling part 70 s reaches the first edge 901 of therelief port 900 as shown in FIG. 3. Thus, the invalid stroke is verysmall from the time the pressure is released when the brake is in anon-operating position to the time the fluid pressure starts to rise.Moreover, in the terminal end of stroke of the braking operation asshown in FIG. 4, the seal coupling part 70 s of the seal ring 70 islocated at the outer peripheral part of the piston 91 (or 92) passingover the tapered surface 951 of the recess 93. As a result, a morereliable sealing performance of the seal coupling part 70 s can beobtained, and a fluid pressure, which is sufficiently high, can begenerated in the fluid pressure chamber 1010 (or 1020).

When the braking operation is stopped by a driver, the piston 91 (or 92)is returned to a state of FIG. 2 under the effect of the returning forceof the return spring 81 (or 82). In the returned state, the sealcoupling part 70 s of the seal ring 70 is brought back again into a formin which the seal coupling part 70 s peeps into the inside of theopening of the relief port 900. In this returned position, i.e., in thenon-operating position of the brake, the base part 700 of the seal ring70 is brought into the attachment groove 42 and the seal coupling part70 s at the inner periphery of the distal end of the inner lip 701 isallowed to slightly project outward from the attachment groove 42.Thereafter, when braking operation is performed again by a driver, theseal coupling part 70 s of the seal ring 70, as in the preceding case,rises the fluid pressure in the fluid pressure chamber 1010 (or 1020)passing over only the first edge 901 of the relief port 900. When thebraking operation is stopped, the seal coupling part 70 s is returned tothe original state passing over only the first edge 901 (without passingover the second edge 902) of the relief port 900.

Moreover, in the master cylinder 10, other elaboration is applied to theform of the seal ring 70 in order to prevent residual pressure. FIG. 7shows an embodiment of a free state before attachment of the seal ring70. As previously mentioned, the seal ring 70 includes a base part 700and an inner and an outer lip 701, 702 extending from the inner andouter peripheral parts of the base part 70 to fluid pressure chamber1010 (or 1020). The outer lip 702 has a taper serving as the tighteningmargin 702 b on the outer peripheral surface, while the inner lip 701has a convex part serving as the seal coupling part 70 s on the innerperiphery of its distal end part. The base part 700 is provided withsecond protrusion parts 7002 which are located at places, for example,six places on the corner part facing the void 75 or piston 91 (or 92)side which places are equidistantly separated in the peripheraldirection, and an axial groove (flow passage) 7003 is formed betweenevery adjacent second projection parts. In other words, the groove (flowpassage) 7003 is formed by notching the corner part of the base part700. Those grooves (flow passages) 7003 smoothly intercommunicate therelief port 900 side and the communication passage 50 (or 52) side bycooperating with the voids 75 or by enlarging the voids 75.

Now, attention should be paid to the side surface of the inside (sidesurface on the side where the inner and outer lips 701, 702 extend) ofthe base part 700 of the seal ring 70. There is a swollen part 7005 overthere which serves to enlarge the axial thickness of the base part 700at the inner lip 701 side compared with at the outer lip 702 side. Theswollen part 7005 occupies the entire inner periphery of the base part700 and axially moves the center of oscillation of the inner lip 701toward the second end part 20 b compared with a case where the swollenpart 7005 is not provided. For example, the axial thickness of the basepart 700 is about 1 mm at its part near the outer lip 702 side, butabout 1.5 mm at its part in the vicinity of the root part of the innerlip 701. Accordingly, owing to a provision of the swollen part 7005, theinner lip 701 is reduced a little more than about 0.5 mm in practicallength of the lip. Moreover, owing to a provision of the integralswollen part 7005, the center of oscillation of the inner lip 701 is,when compared with a case where the integral swollen part 7005 is notprovided, shifted toward the second end part 20 b side and the lip ishardly deformed radially inward. Thus, the seal ring 70 having theswollen part 7005 is not greatly reduced in void 75 when it is affectedby the fluid pressure variation of the surroundings, and the relief port900 side and the communication passage 50 (or 52) are surelycommunicated with each other through the void 75, in a non-operatingposition of the master cylinder 10. Accordingly, the residual pressurecan effectively be prevented from occurring. This makes it possible toeliminate one of the first and second projection parts. The swollen part7005 is formed in the shape of a chevron in section. The swollen part7005 may be changed in height and shape depending on hardness of theseal ring 70. What is important is that much care should be paid so asnot to allow the inner lip 701 to be deformed more than necessary by thevarying fluid pressure and not to overly reduce the void 75.

FIG. 8 shows still another embodiment of a seal ring which is effectivefor prevention of residual pressure. The first and second projectionparts 7006, 7002 formed on the inner peripheral surface of the base part700 shown in FIG. 8 are bulged such that they are generally equal ininside diameter at the base part 700. According to this embodiment,since the first and second projection parts 7006, 7002 are bulged in adotted pattern from the surface of the base part 700, the base part 700is not easily deformed, thus enabling to surely form a flow passagebetween the adjacent projection parts 7006, 7002. For example, eightpieces of the first projection parts 7006 are equidistantly arranged inthe peripheral direction, and sixteen pieces of the second projectionparts 7002 are equidistantly arranged in the peripheral direction. Itshould be noted, however, that the projection parts 7006, 7002 arepreferably not mutually overlapped when viewed in the axial direction,as previously mentioned. The reason is that such an arrangement isdesirable not only from the viewpoint of obtaining an effective flowpassage but also from the viewpoint of shaping a seal ring.

1. A plunger type master cylinder comprising a cylinder housing having acylinder bore axially extending from a first end part which is open anda second end part which is closed and a communication passage which iscommunicated with a reservoir, a piston fitted into said cylinder boreof said cylinder housing, defining a fluid pressure chamber on the sideof said second end part which is closed and axially movable, a seal ringlocated on the side of said cylinder housing, adapted to seal an outerperiphery of said piston, and including an inner lip and an outer lipextending from a base part and an inner and an outer peripheral part ofsaid base part to the side of said fluid pressure chamber, respectively,and a relief port serving as a passage which is located on the side ofsaid piston and adapted to intercommunicate said communication passageand said fluid pressure chamber, an opening of said passage beinglocated at an outer peripheral wall surface of said piston, said plungertype master cylinder having the following features; a recess is formedon an outer peripheral surface of said piston ranging, when axiallyviewed, at least from a second edge on the side of said second end partof said opening of said relief port to a first edge on the side of saidfirst end part of said opening of said relief portion, further rangingto an area away by a predetermined distance from said first edge of saidopening of said relief portion toward said first end part side, andfurther ranging over the entire length in a peripheral direction of saidpiston, said inner lip of said seal ring is smaller in diameter at adistal end part as a free end thereof than at a root part on said basepart side, thereby said seal ring has a seal coupling part situated atan inner periphery of the distal end part of said inner lip which issmall in diameter which is seal-coupled to an outer peripheral wallsurface of said piston and adapted to seal-coupled to an outerperipheral wall surface of said piston, and moreover, a void forcommunicating said relief portion with said communication passagetogether with said recess is formed between an inner periphery of saidinner lip ranging from said seal coupling part to said root part, andsaid seal coupling part of the distal end part of said inner lip of saidseal ring is axially located at the opening part of said relief port onthe side of said piston when said master cylinder is in a non-operatingposition.
 2. The master cylinder according to claim 1, wherein said sealcoupling part of said inner lip of said seal ring traverses the edge ofsaid opening of said relief port only at an area on the side of saidfirst edge so as to be seal coupled to the outer peripheral wall surfaceof said piston which is nearer to said first end part from said firstedge, in accordance with operation of said master cylinder.
 3. Themaster cylinder according to claim 1 further having the followingfeature; said seal ring comprises a plurality of first projection partsformed on an inner peripheral surface of said inner lip between saidroot part and said seal coupling part and bulged from the innerperipheral surface of said inner lip toward the outer peripheral surfaceof said piston, and said first protection parts are spacedly arranged inthe peripheral direction, thereby forming an axial flow passage betweensaid first projection parts which are adjacent to each other in theperipheral direction.
 4. The master cylinder according to claim 3,wherein said seal ring further comprises a plurality of secondprojection parts located on the inner peripheral surface of said innerlip and nearer to the base part from said first projection parts, andsaid second projection parts are also arranged in the peripheraldirection, thereby forming an axial second flow passage between saidsecond projection parts which are adjacent to each other in theperipheral direction.
 5. The master cylinder according to claim 4,wherein said first and second projection parts are arranged such thattheir projection parts are not overlapped with each other when viewed inthe axial direction.
 6. The master cylinder according to claim 1,further having the following features; said seal ring is deformable suchthat said inner lip is oscillated radially inward and outward about saidroot part, said base part, said base part is provided at an insidesurface on the side where said inner and outer lips extend with aswollen part for enlarging the axial thickness of said base part on theinner lip side compared with the outer lip side, and the center ofoscillation of said inner lip is located in a position nearer to thesecond end part side in the axial direction than in a case where noswollen part is provided.
 7. The master cylinder according to claim 1,wherein said seal coupling part is composed of an convex part which isformed over the entire periphery of the distal end part of said innerlip.
 8. The master cylinder according to claim 1, wherein the outerperipheral wall surface of said piston to which said seal coupling partis seal coupled, is located between a part situated in said recessformed in the outer periphery of said piston and a part nearer to saidfirst end part from said recess.
 9. The master cylinder according toclaim 1, wherein said cylinder housing is provided, when viewed in theaxial direction, at a front and a rear part of said recess formed insaid piston with portions for guiding movement of said piston.
 10. Themaster cylinder according to claim 1, wherein an attachment groove forattaching said seal ring is formed in an inner peripheral wall surfaceof said cylinder bore of said cylinder housing, and said seal ring islocated in said attachment groove with said base part sunk in saidattachment groove and with said seal coupling part at the innerperiphery of the distal end part of said inner lip projecting outwardfrom said attachment groove.